1. Field of the Invention
The present invention relates to a railcar primary suspension configured to couple an axle box, which accommodates a bearing for supporting an axle, to a side frame of a truck frame by a connecting mechanism.
2. Description of the Related Art
In a railcar truck, an axle box accommodating a bearing for supporting an axle is supported by an primary suspension in a manner that allows the axle box to be displaced with respect to the truck frame. There are various types of primary suspensions. For example, in the case of an primary suspension of an radius arm type, an axle spring which is a coil spring is interposed between an axle box and a side frame of a truck frame disposed above the axle box, and a tip end portion of a radius arm extending in a vehicle longitudinal direction from the axle box is supported in a displaceable manner by receiving seats extending downward from the truck frame (e.g., see Japanese Patent No. 2799078).
Specifically, as shown in FIG. 10, in the case of a conventional primary suspension 1 of the radius arm type, a shaft 4 is attached via a rubber bushing 3 to a tip end tubular part 2a of an radius arm 2 extending in the vehicle longitudinal direction from an axle box (not shown). Projecting portions 4a are formed at both side surfaces of the shaft 4 in a vehicle width direction, and the projecting portions 4a are fitted from below in fitting grooves 6a of respective receiving seats 6 extending downward from a side frame 5 of a truck frame. One side surface of each projecting portion 4a and one side surface of the corresponding fitting groove 6a, the side surfaces being in contact with each other, are vertical surfaces, and the other side surface of the projecting portion 4a and the other side surface of the fitting groove 6a, the other side surfaces being in contact with each other, are upward-tapered surfaces slightly inclined inward. A plate 7 is fixed by two bolts 8 and 9 to a lower surface of the receiving seat 6 so as to close a lower end opening of the fitting groove 6a. 
Since the other side surface of the projecting portion 4a and the other side surface of the fitting groove 6a, the other side surfaces being in contact with each other, are tapered surfaces, the more the bolts 8 and 9 are fastened, the more the projecting portion 4a is moved upward by the plate 7 within the fitting groove 6a, and thereby the projecting portion 4a is pressed into the fitting groove 6a. Accordingly, the projecting portion 4a is pressed into the fitting groove 6a in a manner similar to inserting a wedge. In this case, considering the strength of the receiving seat 6, it is necessary to set fastening torque of the bolts 8 and 9 to be lower than standard fastening torque of the other bolts (for example, the fastening torque of M12 bolts is 100 Nm and the fastening torque of M16 bolts is 244 Nm; however, the fastening torque of the bolts 8 and 9 for fixing the plate 7 is set to 60 Nm). Therefore, at the time of fastening the bolts 8 and 9, it is necessary to fasten them with torque that is different from the standard torque for fastening the other bolts nearby. This causes low workability and requires proficiency for the work.
Moreover, a predetermined area or larger needs to be secured as a contact area between the projecting portion 4a of the shaft 4 and the fitting groove 6a of the receiving seat 6. This requires performing the following onerous work: first, with the projecting portion 4a of the shaft 4 being fitted in the fitting groove 6a of the receiving seat 6, temporarily fasten the bolts 8 and 9 with torque higher than preset torque to sufficiently secure the contact area between the projecting portion 4a and the fitting groove 6a; thereafter, loosen the bolts 8 and 9; and then fasten the bolts 8 and 9 again with the preset torque. Furthermore, in order to confirm the contact area between the projecting portion 4a and the fitting groove 6a, it is necessary to measure by a gage whether the length, in the vertical direction, of a gap between an upper end surface of the projecting portion 4a and an upper end surface of the fitting groove 6a satisfies a predefined requirement.